How to distinguish between arcane and divine spellcasters?

Question

I'm making a setting where there's a large cultural importance placed on whether you're a divine or arcane caster. I was wondering if detect magic (or any other plausible and simple method) could determine whether the caster in question is divine or arcane without them having cast a spell first.

I'm building the setting for 5e, but I don't think the spell has changed between editions.

An in-world clarification: Imagine an arcane university where divine spellcasters would not be allowed to enter. All spellcasters are met at the door by the porter, who is expected to work out what kind of caster they are. How do you spot a disguised cleric?

Answer 1

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.

Detect Magic detects the presence of magical objects, magical creatures, and anything with an ongoing magical effect on it. It does not detect a caster simply by virtue of them being a caster. Even if the caster was in the middle of casting a spell it's unclear whether they could be detected using Detect Magic.

Similarly, Detect Magic can determine what school of magic a magical effect comes from, but it says nothing about differentiating between arcane and divine. If it could, it would raise further questions, like 'Can Detect Magic determine the difference between a spell cast by a Bard and one cast by a Wizard.' and so forth.

To test whether a caster is arcane or divine is extremely difficult. You could easily introduce a test to check whether someone is a Wizard, for example, by asking them to scribe a scroll into their spellbook and then cast it. However, numerous methods to gain spells from another class exist, like the Magic Initiate feat, or the Knowledge domain, so just asking them to cast an arcane spell wouldn't prove anything.

On the other hand, you're the DM, and it's your setting. You can just give the university a 'Threshold of Detect Arcane/Divine" without having to mechanically back it up. It's perfectly reasonable for a large group of powerful spellcasters with time on their hands to have created something like this, especially in a universe where there are Divination spells that can give you the answers to arbitrary questions.

Answer 2

There's no way to tell what kind of magic a spellcaster has, or even if someone is a spellcaster. But! Crafting a setting isn't just throwing some cultures in a pot, it also involves creating the tools of the world. Magic is a tool, so creating a setting also covers creating new spells or forms of magic that the setting demands.

In such a setting as you describe, spellcasters would have long ago developed a dedicated spell or set of spells specifically to solve this problem of identification. Creating this setting demands that you create the spells they use for this. Homebrew a spell (or two) using detect magic as a template, and that's how people tell them apart.

For example, you might create this:

Almar's echoauric discrimination
1st-level divination (ritual)

Casting time: 1 action
Range: Self
Components: V, S, M (a crystal hourglass blessed by a High Priest of Mystra worth at least 100gp)
Duration: Concentration, up to 10 minutes

In 1481 DR a lowly scribe-mage of Mystra produced this revolutionary spell. Casting it reveals the past usage of magic by a creature, using obscure chronomantic principles rediscovered by Almar in her researches.
You can use your action to focus on a creature to reveal the presence or absence of spellcasting use in the creature's past, perceived as an “chrono-echoic aura.” A second action used in the following round reveals the broad type of the creature's previous spellcasting, whether divine, arcane, or both.

Or you might create two spells, one for each type of magic. You might create variations that are available only to divine or arcane spellcasters, depending on the needs of your cultural setup. And given the existence of such a spell, the door is opened to the existence of magic items that grant it continuously, which would make it more practical for use by a wizardly academy's door porter, a well-heeled socialite, a magistrate, and so on, though still likely uncommon.

Answer 3

I only mention this because of your statement.....

(or any other plausible and simple method)

If this is for the purpose of plot development, or even back story to your world, why not just discriminate based on the casting action?

Holy Symbol.

A holy symbol is a representation of a god or pantheon. It might be an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic. The Player’s Handbook lists many gods in the multiverse and their typical symbols. A cleric or paladin can use a holy symbol as a spellcasting focus, as described in chapter 10. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield.

Arcane casters need to have spell components that are consumed on casting the spell.

Even if someone goes through the trouble of using Meta-magic, it still would show to someone who is familiar with the casting of that spell.

Alternatively, if they try very hard to sneak in, maybe that would lead to an interesting line of consequences after they find themselves surrounded by, soon to be hostile, individuals if they are uncovered.

Answer 4

I think a lot depends on whether you are trying to present your players with a challenge or an obstacle. The level of paranoia and despotism in the setting might also determine what level of intrusion is culturally acceptable and how much effort the university is willing to put forth to keep out the godly.

Probably the easiest approach to set up and calibrate would be some variant on a lie detector. At its simplest, a permanent Zone of Truth in the screening zone could be combined with an appropriately phrased "are you a cleric?" test. To crank things up a notch, the gatekeepers could be experienced in trying to force lies out of people to test whether they made the Charisma save or not. Detect Thoughts could also work with the lie detector routine, particularly in conjunction with questions or statements intended to get a rise out of the religious (blasphemies, etc). In a really authoritarian setting, the university might demand that entrants botch their Wisdom save, allowing the questioner access to the entrant's deeper thoughts. In many settings, Arcane casters are a paranoid lot, so this last option would probably not sit too well with the university community.

With a little more hand-waving, you could probably argue that an arcane focus would accumulate residue from the magic its owner has channeled through it, which Detect Magic could pick up. No recently-used focus? No admission. The same logic might apply to determining whether someone has a divine focus on them. Depending on how you want to play this, you could emphasize the capacity to detect arcane foci but leave the detectability of divine foci as something that the university doesn't publicize. A softer form of this would be to require entrants to pick out a focus out of a lineup of plausible fakes.

Answer 5

The rulebook itself does not provide any way of differentiating, either through Detect Magic or anything else. However, the DM could potentially allow things like Arcana or Religion skill checks to determine the type of magic, or perhaps even the type of spellcaster. Remember, 5e is purposely rules-lite to encourage the DM to come up with ways of doing things.